Differential pressure measurement arrangement utilizing dual transmitters

ABSTRACT

A two-wire transmitter senses differential pressure, absolute pressure, and process temperature of a process fluid. The information can be used to provide an output representative of mass flow through a pipe. The transmitter has an electronics module housing attached to a sensor module housing. The sensor module housing contains all the sensors for the pressures and includes a boss input for receiving a signal representative of a temperature or pressure signal. The three process variables are appropriately digitized, and received by an electronics circuit board in the electronics housing including a microprocessor for calculating the mass flow. The microprocessor in the electronics housing also calculates a compressibility factor and a discharge coefficients according to a polynomials of specific forms. The boss is integral to the sensor module housing and is adapted to fit either shielded twisted pair cabling or conduit.

This is a Continuation of application Ser. No. 08/258,262, filed Jun. 9,1994, now U.S. Pat. No. 5,606,513 which is a Continuation-In-Part ofapplication Ser. No. 08/124,246, filed Sep. 20, 1993, now abandoned.

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure, as it appears in the Patent and TrademarkOffice patent file or records, but otherwise reserves all copyrightrights whatsoever.

REFERENCE

Cross reference is made to co-pending application Ser. No. 08/117,479,filed Sep. 7, 1993 now abandoned, assigned to the same assignee as thepresent application, and entitled "Multivariable Transmitter."

BACKGROUND OF THE INVENTION

This invention relates to a field mounted measurement transmittermeasuring a process variable representative of a process, and moreparticularly, to such transmitters which have a microprocessor.

Measurement transmitters sensing two process variables, such asdifferential pressure and a line pressure of a fluid flowing in a pipe,are known. The transmitters typically are mounted in the field of aprocess control industry installation where power consumption is aconcern. Measurement transmitters provide a current outputrepresentative of the variable they are sensing, where the magnitude ofcurrent varies between 4-20 mA as a function of the sensed processvariable. The current needed to operate a measurement transmitter mustbe less than 4 mA in order for the transmitter to adhere to this processcontrol industry communications standard. Other measurement transmitterssense process grade temperature of the fluid. Each of the transmittersrequires a costly and potentially unsafe intrusion into the pipe, andeach of the transmitters consumes a maximum of 20 mA of current at 12V.

Gas flow computers sometimes include pressure sensing means common to apressure sensing measurement transmitter. Existing gas flow computersare mounted in process control industry plants for precise processcontrol, in custody transfer applications to monitor the quantity ofhydrocarbons transferred and sometimes at well heads to monitor thenatural gas or hydrocarbon output of the well. Such flow computersprovide an output representative of mass flow rate as a function ofthree process variables. The three process variables are thedifferential pressure across an orifice plate in the pipe conducting theflow, the line pressure of the fluid in the pipe and the processtemperature of the fluid. Many flow computers receive the three requiredprocess variables from separate transmitters, and therefore include onlycomputational capabilities. One existing flow computer has two housings:a first housing which includes differential and line pressure sensorsand a second transmitter-like housing which receives an RTD inputrepresentative of the fluid temperature. The temperature measurement issignal conditioned in the second housing and transmitted to the firsthousing where the gas flow is computed.

Methods of measuring natural gas flow are specified in Orifice Meteringof Natural Gas and other Related Hydrocarbon Fluids, Parts 1-4, which iscommonly known as AGA Report No. 3. Calculating the mass flow raterequires that the compressibility factor for the gas and the orificedischarge coefficient be computed. The compressibility factor is thesubject of several standards mandating the manner in which thecalculation is made. Computing the compressibility factor according tothese standards expends many instruction cycles resulting in asignificant amount of computing time for each calculation of mass flowand a large power expenditure. Accordingly, the amount of time betweensubsequent updates of the mass flow rate output is undesirably long ifeach update is calculated from a newly computed compressibility factor,so as to slow down a process control loop. Even if the compressibilityfactor is calculated in the background so as to prevent lengthening theupdate rate, the mass flow rate output is calculated from a stalecompressibility factor which provides poor control when the processchanges rapidly. Furthermore, calculation of the compressibility factorentails storage of large numbers of auxiliary constants which alsoconsumes a large amount of power. AGA Report No. 3 Part 4 mandates massflow rate accuracy of 0.005%, resulting either in slow update times, useof stale compressibility factors in computing mass flow rate or powerconsumption greater than 4 mA. Similarly, direct calculation of theorifice discharge coefficient requires raising many numbers tonon-integer powers, which is computationally intensive for low powerapplications. This also results in undesirably long times betweenupdates or power consumption greater than mandated by the 4-20 mAindustry standard.

There is thus a need for a field mounted multivariable transmitteradaptable for use as a gas flow transmitter having improved updatetimes, but consuming less than 4 mA at 12 V of power without sacrificingthe accuracy of the calculation.

Another aspect of the present invention relates to pressure measurementdevices, and particularly to pressure transmitter systems that respondto pressure at least two discrete locations and that communicate with aseparate controller over a two-wire link.

Pressure transmitters having a transmitter housing that includes adifferential pressure ("ΔP") transducer fluidically coupled to twopressure ports in the housing, are known. Such transmitters furtherinclude in the transmitter housing circuitry coupled to the transducerand communicating the measured ΔP to a distant controller over atwo-wire link. The controller energizes the circuitry over the two-wirelink. Fluid conduits such as pipes or manifolds carry a process fluid tothe transmitter pressure ports. Typically, process fluid immediatelyupstream and downstream of an orifice plate is routed to the respectiveports, such that the ΔP measured by the transducer is indicative ofprocess fluid flow rate through the orifice plate.

In some applications it is desired to measure differential process fluidpressure at locations separated from each other by a distance muchgreater than the scale size of the transmitter housing. To make such ameasurement it is known to attach to the above described ΔP transmitterflexible oil-filled capillary tubes or impulse piping to fluidicallytransmit the process fluid pressures to the housing pressure ports.However, such arrangements suffer from errors due to differences inheight and temperature of the oil-filled capillary tubes.

It is also known to provide a separate pressure transmitter at each ofthe two process fluid measurement locations, and to electrically coupleeach of the pressure transmitters to a "hydrostatic interface unit"(HIU) The HIU communicates with the distant controller over a two-wirelink, and is powered by a separate unit over a different electricallink. The HIU, in turn, electrically powers and communicates with thepressure transmitters, and performs multiple arithmetic operations onthe measured pressures. For example, where the pressure transmitters aremounted on a storage tank of process fluid, the HIU can communicate overthe two-wire link a 4-20 mA signal indicative of the process fluiddensity ρ: ##EQU1## where ΔP is the process fluid pressure differencebetween the transmitters, g is gravitational acceleration, and z is the(user-programmed) vertical separation of the fluid measurementlocations. This system avoids problems associated with oil-filledcapillaries external to the transmitter housing, but has disadvantagesof its own such as the need to mount additional electronic devicesproximate the measurement site and the need for a separate power supplyfor the HIU due in part to the large number of calculations performed bythe HIU.

BRIEF SUMMARY OF THE INVENTION

The present invention is a transmitter for calculating mass flow rate ofa process fluid in a single unit and having low power consumption. Atwo-wire transmitter sensing process variables representative of aprocess includes an electronics module housing attached to a sensormodule housing. The sensor module housing has a pressure sensor forsensing a pressure process variable representative of the process andhas a boss for receiving a signal representative of a second processvariable, such as a temperature signal. The transmitter includesappropriate digitizing circuits for the sensed process variables. Theelectronics housing includes an electronics circuit board having amicroprocessor for calculating the mass flow of the fluid through thepipe, and the board also includes electronics for formatting the processvariables and for coupling the process variables onto the two-wirecircuit. The microprocessor in the electronics housing also calculates acompressibility factor and discharge coefficient according topolynomials of specific forms. A boss is located on the sensor modulehousing and adapted to fit either shielded twisted pair cabling orconduit.

According to another aspect of the invention, a pressure measurementsystem includes a transmitter housing including an internal pressureport. A pressure transducer in the housing couples to the pressure portand provides a signal related to pressure to circuitry in thetransmitter. The circuitry also receives a nonfluidic signal from anexternal pressure transducer coupled to a remote pressure port externalto the transmitter housing. Electronics in the housing process thesignals and provide an output related to pressure at the internal portand at the external port.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing of the present invention connected to a pipe forsensing pressure and temperature therein;

FIG. 2 is a block drawing of the electronics of the present invention;

FIG. 3A-B are curves of the compressibility factor as a function ofpressure at various temperatures for two fluids;

FIG. 4 is a modified cross sectional drawing, showing areas of interest,for the present invention; FIG. 4A is a section of the boss and platetaken along lines 4A--4A; and

FIG. 5 is a cross sectional drawing of the present invention shown witha conduit adapted connector.

FIG. 6 is an elevational view, partially in block diagram and partiallyin section, of an arrangement for measuring differential pressure inaccordance with the invention;

FIG. 7 is a sectional view, partially in block diagram, of a masterpressure transmitter in accordance with the invention;

FIG. 8 is a sectional view, partially in block diagram, of an alternatemaster pressure transmitter in accordance with the invention;

FIGS. 9A and 9B are sectional views, partially in block diagram, ofslave pressure transmitters in accordance with the invention; and

FIG. 10 is an electrical block diagram of the differential pressuremeasurement system of FIG. 6.

For brevity and ease of discussion, items in some figures bear the samereference numeral as items in earlier figures. Such items bearing thesame reference numeral serve the same or similar function.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a multivariable transmitter 2 mechanically coupled to apipe 4 through a pipe flange 6. A flow of natural gas flows through pipe4. In the invention, transmitter 2 receives differential pressure,absolute pressure and temperature, and provides a multivariable outputincluding mass flow rate with reduced power consumption.

A 100 ohm RTD (resistive temperature device) temperature sensor 8 sensesa process temperature downstream from the flow transmitter 2. The analogsensed temperature is transmitted over a cable 10 and enters transmitter2 through an explosion proof boss 12 on the transmitter body.Transmitter 2 senses differential pressure, absolute pressure andreceives an analog process temperature input, all within the samehousing. The transmitter body includes an electronics housing 14 whichscrews down over threads in a sensor module housing 16. Transmitter 2 isconnected to pipe 4 via a standard three or five valve manifold. Whentransmitter 2 is connected as a gas flow computer at a remote site,wiring conduit 20, containing two-wire twisted pair cabling, connectsoutput from transmitter 2 to a battery box 22. Battery box 22 isoptionally charged by a solar array 24. In operation as a data logginggas flow computer, transmitter 2 consumes approximately 8 mA of currentat 12V, or 96 mW. When transmitter 2 is configured as a high performancemultivariable transmitter using a suitable switching power supply, itoperates solely on 4-20 mA of current without need for battery backup.This is achieved through reduction techniques discussed below. Switchingregulator circuitry (not shown) ensures that transmitter 2 consumes lessthan 4 mA.

In FIG. 2, a metal cell capacitance based differential pressure sensor50 senses the differential pressure across an orifice in pipe 4. Asilicon based strain gauge pressure sensor 52 senses the line pressureof the fluid in pipe 4, and 100 ohm RTD sensor 8 senses the processtemperature of the fluid in pipe 4 at a location typically downstreamfrom the differential pressure measurement. A low cost silicon based PRT56 located on a sensor analog board 68 senses the temperature proximateto the pressure sensors 50,52 and the digitized output from sensor 56compensates the differential and the line pressure. Analog signalconditioning circuitry 57 filters output from sensors 8,50 and 52 andalso filters supply lines to a set of A/D circuits 58-64. Four low poweranalog to digital (A/D) circuits 58-64 appropriately digitize theuncompensated sensed process variables and provide four respective 16bit wide outputs to a shared serial peripheral interface bus (SPI) 66 atappropriate time intervals. A/D circuits 58-64 are voltage orcapacitance to digital converters, as appropriate for the input signalto be digitized, and are constructed according to U.S. Pat. Nos.4,878,012, 5,083,091, 5,119,033 and 5,155,455, assigned to the sameassignee as the present invention. Circuitry 57, PRT 56 and A/D circuits58-64 are physically situated on analog sensor board 68 located insensor housing 16.

Microprocessor 72 compensates sensed and digitized process variables. Asingle bus 76 communicates compensated process variables between thesensor housing and electronics housing 14. A second microprocessor 80 inelectronics housing 14 computes installation specific parameters as wellas arbitrating communications with a master unit (not shown). The dualmicroprocessor structure of transmitter 2 doubles throughput compared toa single microprocessor unit having the same computing function, andreduces the possibility of aliasing. Aliasing is reduced in the dualmicro structure, since it allows the process variable to be convertedtwice as often as a single microprocessor transmitter with the sameupdate rate. In other words, since compensation and computation isfunctionally partitioned, the processor 80 does not interleavecalculation intensive compensation task with the application andcommunications task. In transmitter 2 sensor microprocessor 72 providescompensated process variables while the electronics microprocessor 80simultaneously computes the mass flow using compensated processvariables from the previous update period. For example, one installationspecific physical parameter is mass flow when transmitter 2 isconfigured as a gas flow transmitter. Alternatively, transmitter 2includes suitable sensors and software for turbidity and levelmeasurements when configured as an analytical transmitter. Finally,pulsed output from vortex or turbine meters can be input in place of RTDinput (and the digitizing circuitry appropriately altered) and used incalculating mass flow. In various embodiments of the presentmultivariable transmitter invention, combinations of sensors(differential, gauge, and absolute pressure, process temperature andanalytical process variables such as gas sensing, pH and elementalcontent of fluids) are located and are compensated in sensor modulehousing 16.

During manufacture of transmitter 2, pressure sensors 50,52 areindividually characterized over temperature and pressure and appropriatecorrection constants are stored in electrically erasable programmableread only memory (EEPROM) 70. Microprocessor 72 retrieves thecharacterization constants stored in EEPROM 70 and calculates polynomialto compensate the digitized differential pressure, relative pressure andprocess temperature. Microprocessor 72 is a Motorola 68HC05C8 processoroperating at 3.5 volts in order to conserve power. Sensor digital board76 is located in sensor housing 16 and includes EEPROM 70, micro 72 andclock circuit 74. The functionality on boards 67 and 68 may be combinedthrough ASIC technology into a single sensor electronics board. Bus 76includes power signals, 2 handshaking signals and the three signalsnecessary for SPI signalling. A clock circuit 74 on sensor digital board67 provides clock signals to microprocessor 72 and to the A/D circuits58-64.

A Motorola 68HC11F1 microprocessor 80 on output circuit board 78arbitrates communications requests which transmitter 2 receives over atwo-wire circuit 82. When configured as a flow computer, transmitter 2continually updates the computed mass flow. All the mass flow data islogged in memory 81, which contains up to 35 days worth of such data.When memory 81 is full, the user connects gas flow computer 2 to anothermedium for analysis of the data. When configured as a multivariabletransmitter, transmitter 2 provides the sensed process variables, whichincludes as appropriate differential pressure, absolute pressure andprocess temperature.

As discussed above, prior art techniques for calculating mass flow rateare very complex and have large power requirements due to themicroprocessor and memory requirements. In the past, reducing powermeans reducing accuracy of the mass flow rate calculation. The inventionovercomes this limitation by characterizing these complex equations aspolynomials and storing the coefficients of the polynomials innon-volatile memory. The microprocessor retrieves the coefficients for afluid at its temperature and calculates mass flow using the simpler (andhence less power intensive) polynomial.

Microprocessor 80 calculates the computation intensive equation for massflow rate, given as: ##EQU2## where: C_(d) =coefficient of discharge forflange-tapped orifice meter,

d=orifice plate bore diameter, in inches, calculated at flowingtemperature (T_(f)),

E_(V) =velocity of approach factor,

G_(r) =real gas relative density (specific gravity) at standardconditions,

h_(w) =orifice differential pressure, in inches of water at 60 degreesF.,

P_(f1) =flowing pressure at upstream tap, in pounds force per squareinch absolute,

q_(v) =mass flow rate, in standard cubic feet per hour,

T_(f) =flowing temperature, in degrees Rankine,

Y₁ =expansion factor (upstream tap),

Z_(s) =compressibility factor at standard conditions (P_(s), T_(s)), and

Z_(f1) =compressibility factor at upstream flowing conditions (P_(f1),T_(f)).

There are a number of standards for calculating gas compressibilityfactor. The American Gas Association (AGA) promulgated a standard in1963, detailed in "Manual for the Determination of SupercompressibilityFactors for Natural Gas", PAR Research Project NX-19. In 1985, the AGAintroduced another guideline for calculating the compressibility factor,"Compressibility and Supercompressibility for Natural Gas and otherHydrocarbon Gases," AGA Transmission Measurement Committee Report No. 8,and in 1992 promulgated "Compressibility Factors of Natural Gas andother Related Hydrocarbon Gases," AGA Report No. 8, for the samepurpose.) In AGA Report No. 8 (1992), the compressibility factors, Z_(s)and Z_(f1), are defined as: ##EQU3## where B is a second virialcoefficient, K is a mixture size parameter, D is a reduced density,C_(n) are coefficients which are functions of composition, T is theabsolute temperature, and each of the constants include auxiliaryconstants defined in AGA Report No. 8. Curves of the compressibilityfactor as a function of pressure at various temperatures are given inFIGS. 3A-B, respectively for 100% methane gas and natural gas with ahigh carbon dioxide content. Direct calculation of the compressibilityfactors Z_(s) and Z_(f1) is very computationally intensive when a fluidcontains a large number of constituents. Microprocessor 80 calculatesthese compressibility factors using coefficients derived from leastsquares minimized techniques. As the number of fluids contemplated foruse with the present invention is large, and the magnitude of thecompressibility factor varies significantly, it is preferable to usepolynomials of the form: ##EQU4## where A_(ij) is a curve fittingderived constant stored in EEPROM 70, T is the process temperature and Pis the absolute pressure, and where i and j take on integer valuesbetween -9 and 9, depending on the AGA standard used to calculate thecompressibility factor. A 63 term polynomial suffices for mostapplications. Polynomials of this form and number of terms reduce theamount of computation over direct calculation methods, thereby reducingthe time between updates of the mass flow output and the operating powerrequirements of transmitter 2. Moreover, such a technique obviates alarge memory to store great numbers of auxiliary constants, again savingpower.

The discharge coefficient, C_(d), is also very computationally intensiveand is given for pipe diameters smaller than 2.8 inches and given by:##EQU5## for pipe diameters greater than 2.8 inches, the dischargefactor is given by: ##EQU6## where β=d/D, d is the orifice borediameter, D is the pipe internal diameter, R_(D) is the Reynolds numbergiven by R_(D) =ρVD/μ, where ρ is the fluid density, V is the averageflow velocity in the pipe and μ is the fluid viscosity. As with thecompressibility factor, the discharge factor is preferably curve fit,but using polynomials of the form, ##EQU7## where b_(j) is calculatedempirically and β is as previously defined. Polynomials of this formreduce the amount of computation over direct calculation methods,reducing the time between updates of the mass flow output and theoperating power requirements of transmitter 2.

Transmitter 2 has a positive terminal 84 and a negative terminal 86, andwhen configured as a flow computer, is either powered by battery whilelogging up to 35 days of mass flow data, by a conventional DC powersupply. When transmitter 2 is configured as a high performancemultivariable transmitter, terminals 84,86 are connected to twoterminals of a DCS controller 88 (modelled by a resistor and a powersupply). In this mode, transmitter 2 communicates according to a HART®communications protocol, where controller 88 is the master andtransmitter 2 is a slave. Other communications protocols common to theprocess control industry may be used, with appropriate modifications tomicroprocessor code and to encoding circuitry. Analog loop currentcontrol circuit 100 receives an analog voltage signal from a digital toanalog converter in an ASIC 104 and provides a 4-20 mA current outputrepresentative of any of the process variables. HART® receive circuit102 extracts digital signals received from controller 88 over two-wirecircuit 82, and provides the digital signals to ASIC 104 whichdemodulates such signals according to the HART® protocol and alsomodulates digital signals for transmission onto two-wire circuit 88.Circuit 104 includes a Bell 202 compatible modem.

A clock circuit 96 provides a real time clock signal to log absolutetime corresponding to a logged mass flow value. Optional battery 98provides backup power for the real time clock 96. When transmitter 2 isconfigured as a multivariable transmitter, power intensive memory 81 isno longer needed, and the switching regulator power supply is obviated.Diodes 90,92 provide reverse protection and isolation for circuitrywithin transmitter 2. A switching regulator power supply circuit 94, ora flying charged capacitor power supply design, provides 3.5V and otherreference voltages to circuitry on output board 78, sensor digital board67 and sensor analog board 68.

In FIG. 4, sensor housing 16 of measurement transmitter 2 is shown withboss 12 in detail, along with a hexagonally shaped cable retainer 150.Boss 12 is adaptable for use with cables carrying both analog anddigital signals representative of a process variable. Although acylindrical bulkhead protruding from sensor housing 16 is shown, thepresent invention is practicable with a flush signal input. Furthermore,boss 12 is shown as integral to housing 16, but can be screwed in, laserwelded or otherwise joined. Armored cabling 152 includes 4 signal wires154 for a 4 wire resistive measurement, but may include other numbers ofsignal wires as appropriate. Armored cabling 152 has a conductive shield155 protecting signal wires 154 from EMI interference and terminates ina rubber plug 156 having a grounding washer 158 with copper groundingtape 157. Shield 155 is electrically connected to grounding washer 158with copper tape 157. Two guide sockets 163 and four signal connectorsockets 167 mate to guidepins 165 and feedthroughs 164 in a groundedplate 160 which is welded into boss 12. Plate 160 is preferablyfashioned out of stainless steel to resist corrosive environments. Thearmored cable assembly comprising armored cable 152, rubber plug 156,washer 158, sockets 167 and 163, copper tape 157, is mated to groundedplate 160 in bulkhead 12 and then threaded hex retainer 150 slides overthe cable assembly and is screwed into the straight inner diameterthreads of bulkhead 12. The straight threads on boss 12 stress isolatehousing 16 from stresses induced by 1/2" NPT conduit, which undesirablyaffect the accuracy of the sensed pressure process variables.

In back of plate 160, feedthrough pins 164 connect to optionalelectrostatic and EMI filters 166, designed to minimize interferencefrom electrically noisy field locations. Feedthrough pins 164 are pottedin glass so that grounded plate 160 seals the interior of transmitter 2from the environment. As transmitter 2 may be mounted in areas wherehazardous and/or explosive gases are present, an explosion proof clamp168 fits between a groove 170 in boss 12 and a screw hole 172 in hexretainer 150. A screw 174 securely fastens explosion proof clamp 168 inplace. When the present invention is mounted in explosion proofinstallations, hex retainer 150 is replaced by an conduit connector 180as shown in FIG. 5. Connector 180 has inner diameter threads adapted toreceive 1/2 inch conduit commonly used in the process control industry.Explosion proof clamp 168 may also be used with this adaptation of thepresent invention. The location of boss 12 as integral to sensor modulehousing 16 is preferred since the signal does not travel through theelectronics housing where noisy digital signals are present. Rather,such a location minimizes the distance which the uncompensatedtemperature signal must travel before digitization by sensor micro 72.Furthermore, a direct connection to the electronics housing could allowcondensation to enter the housing. Entering through the sensor moduleprovides modularity between units because the compensation and signalconditioning steps are performed in the same sensor module. The dualmicroprocessor structure coupled with the boss 12 on sensor module 16provides reduced power consumption for the three process variablemeasurement, reduces the compensation errors in each of the threevariables and provides a smaller housing with less weight than existingtransmitters designed with mass flow rate outputs.

In FIG. 6, differential pressure measurement system 210 includes a"master" pressure transmitter 212 and a "slave" pressure transmitter214. Pressure transmitters 212,214 bolt to flanges 216,218,respectively, at ports 220,222 on storage tank 224. Tank 224 holds aprocess fluid (not shown). System 210 measures a hydrostatic pressuredifferential of the process fluid between ports 220,222. The distancebetween ports 220,222 is on the order of or greater than the size of oneof the transmitters 212,214, such that the measurement cannot be madewith a single transmitter unless oil-filled capillary tube extensions orimpulse piping are used. Each of the transmitters 212,214 includes apressure transducer and, preferably, preconditioning electronics toprovide an electrical output indicative of the process fluid pressure atthe respective port 220,222. Transmitters 212,214 can measure anabsolute pressure, a differential pressure, or (as shown) a gaugepressure of the process fluid at the respective ports 220,222, butpreferably they make the same type of measurement to reduce atmosphericpressure effects.

Slave transmitter 214 conveys to master transmitter 212 an electricalrepresentation of the process fluid pressure at port 222 via electricalconnection 226. Connection 226 can comprise a shieldedmultiple-conductor cable with standard multi-pin electrical connectorsaffixed at both ends, or it can comprise bendable tubular conduit withone or more wires running therethrough. Such conduit protects and, if itis electrically conductive, electrically shields the wire or wires fromelectromagnetic interference.

Master transmitter 212, in addition to measuring the process fluidpressure at port 220, calculates a process fluid pressure differencebetween ports 220 and 222 by calculating a difference between thepressure measurements made by transmitters 212,214. If pressuretransmitters 212,214 are configured for gauge pressure measurement, thecomputed difference between their outputs will include a contributiondue to the atmospheric pressure difference between the two pressuretransmitter locations. This atmospheric contribution can be correctedfor by an offset adjustment within master transmitter 212, or, dependingupon desired system accuracy and vertical separation of transmitters212,214, can be ignored.

Control system 230 sends commands to and receives signals from mastertransmitter 212 over two-wire link 228 (preferably in a HART® format,available from Rosemount Inc., Eden Prairie, Minn., U.S.A.), and mastertransmitter 212 can, if desired, communicate in like manner with slavetransmitter 214. Control system 230 energizes master transmitter 212over link 228, and master transmitter 212 in turn energizes slavetransmitter 214 over connection 226. Preferably, master transmitter 212adjusts the electrical current flowing through link 228 between 4 mA and20 mA as an indication of the calculated process fluid pressuredifference.

Master pressure transmitter 212 is shown in greater detail in FIG. 7.For clarity, the portion of the transmitter housing above line 213--213is shown rotated 90° relative to transmitter housing portions below line213--213. A pressure transducer 232, preferably a capacitive cell asdescribed in U.S. Pat. Nos. 4,370,890 and 4,612,812, responds to adifference in pressure between process fluid at pressure port 234 andambient air at pressure port 236. As shown, transducer 232 couples tothe pressure ports via isolator diaphragms 238,240 and passageways242,244 filled with, for example, silicone oil. Pressure transducer 232can alternately measure absolute pressure of process fluid at port 234,in which case port 236, diaphragm 240, and passageway 244 can beeliminated. Measurement circuitry 246 couples to transducer 232 by wires245, and provides a first pressure output P₁ on link 248 responsive tothe relative or absolute pressure at port 234. Link 248, and otherelectrical connections in the figures, are drawn with a thickened lineto make it clear that they can comprise multiple independent conductors.Preferably, circuitry 246 includes a thermistor or other temperaturesensor (see FIG. 10), which is in close thermal communication withtransducer 232 and which is used by circuitry 246 to compensate forthermal characteristics of transducer 232. Hence, first pressure outputP₁ on link 248 has reduced sensitivity to temperature variations atmaster transmitter 212.

Advantageously, master transmitter 212 includes ΔP calculation circuitry250 which receives the first pressure output P₁ over link 248 and asecond pressure output P₂ over link 248', and calculates therefrom thepressure difference ΔP=P₂ -P₁. Measurement P₂ is indicative of therelative or absolute pressure at port 234', and, like P₁, is temperaturecompensated. Circuitry 250 then communicates the pressure difference ΔPover link 228 through communication port 252 in transmitter 212 housingto control unit 230. In the embodiment shown in FIGS. 6 and 7, P₁ and P₂are themselves both differential pressure measurements since they areindicative of gauge pressure. Circuitry 250 also serves to powercircuitry 246 over link 248 and corresponding circuitry 246' in slavetransmitter 214 (see FIG. 9a) over link 248'. Use of the dualtransmitters 212,214 and inclusion of ΔP calculation circuitry 250 inmaster pressure transmitter 212 eliminates the need for externaloil-filled capillaries, as well as the need for a separate computationalunit or the need for control unit 230 to perform such calculations.

FIG. 8 shows an alternative master transmitter 260 similar to mastertransmitter 212 of FIG. 7, with similar items bearing the same referencenumber. The boss 262 near the base of transmitter 212, which comprised adedicated communication port to receive the electrical signal indicativeof pressure, has been eliminated in transmitter 260. Instead, circuitry50 couples to slave transmitter 214 over wires 264 which enter thetransmitter housing through one of the two standard communication portsat the top of the transmitter (see ports 252,253 of transmitter 212 inFIG. 6). Wires 228,264 couple to circuitry 250 via terminal block 266and feedthroughs which penetrate the transmitter housing wall. Byeliminating the need for boss 262 and for a dedicated cable connection226, a differential pressure system incorporating transmitter 260 ratherthan transmitter 212 can be made at a reduced cost.

FIG. 9a shows slave pressure transmitter 214 from FIG. 6 in greaterdetail. Primed reference numerals identify components having the samefunction as previously discussed components having correspondingunprimed reference numerals. Primes (') have been added to associate thenumbered component with slave pressure transmitter 214. Advantageously,slave transmitter 214 uses a pressure transmitter 232' and measurementcircuitry 246' substantially the same as corresponding transmitter 232and circuitry 246 of master transmitter 212 or 260. Such duplication ofparts reduces manufacturing inventory and lowers cost. Connection 226enters slave pressure transmitter 214 through a sole communication port268. Connection 226 terminates in a multiple-pin connector affixed atits end, which reversibly joins to a mating member 270, thereby tocomplete the electrical link 248'.

FIG. 9b shows an alternative slave transmitter 272 which uses a terminalblock 274 and communication ports 276,278 in place of port 268 andmating member 270 from transmitter 214. Such substitution permits thecustomer to use standard metal conduit with feedthrough wires to connectthe slave transmitter to the master transmitter. Slave transmitter 272can be used with master transmitter 260 as a differential pressuremeasurement system. Measurement circuitry 246', discussed above, isshown as a pair of circuit boards coupled together coupled to transducer280 through ribbon cable 245'. Transmitter 272 comprises pressuretransducer 280, which measures the absolute pressure of the processfluid at pressure port 234'.

FIG. 10 is an electrical block diagram of the differential pressuremeasurement system shown in FIGS. 6, 7, and 9a. System 210 includescalculation circuitry 250 coupled to transducers 232 and 232'. FIG. 10shows measurement circuitry 246 in more detail. Circuitry 246 couplesvia lines 245 to capacitors 290 and 292 in transducer 232. Capacitors290 and 292 can be configured to measure differential pressure.Circuitry 246 includes a resistance temperature device (RTD) 298 coupledto measurement input circuitry 300 which also couples to capacitors 290and 292 of transducer 232. Analog-to-digital converter 304 selectivelycouples to transducer 232 or RTD 298 through multiplexer 302 andcircuitry 300. Analog-to-digital converter 304 couples to microprocessor306 which also connects to memory 308. Memory 308 contains variousinformation including information regarding zero and span, and variouscoefficients for correction of, for example, nonlinearity of transducer232 output with pressure and variation of transducer 232 output withtemperature. Microprocessor 306 communicates with calculation circuitry250 over line 248, providing a pressure output P1 as a function oftransducer 232 output adjusted by the zero and span values and correctedby the correction coefficients together with the RTD 298 output.Circuitry 250 can program the contents of memory 308 over line 248.

Circuitry 250 includes difference circuit 312, microprocessor 314 andmemory 316. Microprocessor 314 couples to circuitry 246 and 246',difference circuit 312, memory 316, current control 318, and serialinterface 320. Difference circuit 312 also receives the outputs of 246and 246'. Microprocessor 314 communicates with circuitry 246,246'through connections 248,248'. Microprocessor 314 controls microprocessor306 to configure circuitry 246. Further, pressure information isprovided directly to microprocessor 314 and pressure differential ΔP isprovided to microprocessor 314 through difference circuit 312.Microprocessor 314 communicates over two-wire link 228 and controls thecurrent flowing through loop 228 using current control circuitry 318 inresponse to measured pressure values. Serial interface 320 is used fordigital communications over current loop 228.

Microprocessors 306 and 306' in circuitry 246 and 246', respectively,perform correction and compensation functions on the pressure sensed bysensors 232 and 232', respectively. Microprocessors 306,306' usecorrection coefficients stored in memory 308,308'. Thus, units 246,246'are easily interchangeable and can be individually calibrated duringmanufacture.

Typical prior art schemes for measuring pressure from a remote locationwhich is separated from the transmitter use a small capillary filledwith oil to communicate with the remote transducer, as described in theBackground section.

The present invention offers a number of advantages over the prior art.Sensor measurements from a remote location are immediately convertedinto an electrical signal. The electrical signal can be compensated atthe remote location whereby the signal provided to the transmitter has ahigh level of accuracy. In operation, the system shown in FIG. 10communicates with circuits 246 and 246' over connections 248 and 248'.As shown in FIG. 7, circuitry 246 and transducer 232 reside intransmitter 212. Circuitry 246' and transducer 232' reside in a separateenclosure, separated from transmitter 212. In the embodiment shown inFIG. 6, circuitry 246' resides in slave transmitter 214. Note thatalthough unit 214 has been described as a "transmitter," unit 214 maycomprise any type of remote transducing equipment which provides anelectrical, or other non-fluidic, output signals to transmitter 212.

Circuitry 250 also provides various alarms. Circuitry 250 sends a "HI"alarm condition signal to control unit 230 by causing the signal onwires 228 to exceed a normal range and sends a "LO" alarm conditionsignal by causing the signal to fall below a normal range. The alarm canbe triggered by circuitry 250 for a number of conditions including theoccurrence of P1, P2 or ΔP falling outside of a predetermined range.This information is used to set a warning condition by forcing the loopcurrent to a saturated high or low value. Other parameters could beexamined for warning conditions, such as density.

Further, the circuitry of system 210 not only provides zero, span, andcorrection coefficients individually for pressures P1 and P2 via memory306 and 306', respectively, it can also provide zero, span, andlinearization and temperature correction coefficients for output ΔP viamemory 316. Power reduction may be achieved by multiplexing signalscarried by lines 248,248'. In a typical operation, the entire system canbe powered by a 4 mA signal and 12 volts received from current loop 228.Although capacitive pressure sensors are shown, other types of pressuretransducers can be used such as strain gages. Further, the variouselectrical connections shown can be replaced with optical connections.For example, the connection between circuitry 250 and circuitry 246' canbe one or more optical fibers.

In one embodiment of the invention shown in FIGS. 6 through 10, mastertransmitter 212 measures differential pressure across an orifice in aflow tube while slave transmitter 214 is positioned along the flow tube,upstream or downstream from transmitter 212, and measures absoluteprocess fluid pressure.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges can be made in form and detail without departing from the spiritand scope of the invention. For example, in addition to temperature andpressure sensors, other sensors and sensor inputs can be used with theinvention, such as pH, volumetric or mass flow, conductivity, or gascomposition.

What is claimed is:
 1. A two-wire transmitter in a process controlsystem for sensing process variables representative of a process,comprising:a module housing; a pressure sensor coupled to the modulehousing for sensing a pressure of the process and providing a sensorpressure output; a process variable input for receiving a processvariable signal from an external process variable sensor located outsideof a module housing; an analog to digital converter coupled to thepressure sensor providing a digital pressure output representative ofthe sensor pressure output; compensation circuitry in the module housingreceiving the digital pressure output and providing a compensatedoutput; an electronics housing coupled to the module housing; amicroprocessor in the electronics housing coupled to the compensationcircuitry for receiving the compensated output from the compensationcircuitry and the process variable signal and responsively calculating aphysical parameter related to the process; and output circuitry coupledto a two-wire process control loop for receiving the calculated physicalparameter from the microprocessor and responsively transmitting thephysical parameter on the two-wire process control loop.
 2. Thetransmitter of claim 1, where the physical parameter is mass flow rateand where the pressure sensor senses a differential pressurerepresentative of the process, and where the sensor module housingfurther comprises a second sensor sensing a line pressure representativeof the process and a third sensor sensing a process temperaturerepresentative of the process.
 3. The two-wire transmitter of claim 2,wherein the microprocessor calculates mass flow rate of process fluid asa function of coefficients stored in a memory.
 4. The transmitter ofclaim 1, including a boss in the transmitter coupled to the processvariable input.
 5. The transmitter of claim 4, where the boss isintegral to the transmitter.
 6. The transmitter of claim 4, where theboss is welded into the transmitter.
 7. The transmitter of claim 4,where the boss is screwed into the transmitter.
 8. The transmitter ofclaim 4, where the boss includes a groove on its outer diameter.
 9. Thetransmitter of claim 4, where the boss has straight threads and anadapter screws into the boss.
 10. The transmitter of claim 4, where agrounded plate is welded into the boss.
 11. The transmitter of claim 1,where a retainer threads into the process variable input, and theretainer secures a four wire cable.
 12. The transmitter of claim 1,where a retainer threads into the process variable input and theretainer has a threaded inner diameter for connecting to conduit. 13.The transmitter of claim 1, including circuitry coupled to the two-wireprocess control loop to receive power from the loop to completely powerthe transmitter.
 14. The transmitter of claim 1, wherein thecompensation circuitry comprises a microprocessor.
 15. The transmitterof claim 1, wherein the process variable input is coupled to the modulehousing.